The regional nuclear balance has been disrupted since the North Korean missile program. This in turn led to Japan and yes even South Korea hedging their nuclear bets, "just in case". If one looks at South Korean ballistic missiles and thinks they're not a dual use asset is simply blind.and allow Japan to completely disrupt the regional nuclear balance.
Well, in actuality East Asia was strategically destabilized since the PRC got access to nuclear devices, but that's another matter.
Either way, just how the PRC and DPRK developed their stockpile out of a sense of being in danger from Washington (or the USSR), if the threat level becomes too overt and the US is deemed to unreliable, Japan and the ROK will pursue similar developments, albeit more advanced. The DPRK couldn't be stopped, it's rather amusing to see people think the ROK or Japan could be stopped if they ever felt the urgent need to establish a nuclear deterrent.